1. Field of the Invention
The present invention relates to the field of buildings utilized for manufacturing products and, more particularly, to a facility for fabricating semiconductor devices.
2. Prior Art
Semiconductor devices, which include integrated circuits, are manufactured in "clean room" environments in order not to contaminate the devices being fabricated. These fabrication facilities are commonly referred to as "fabs" and their degree of cleanliness is designated by a rating standard. These standards are defined relevant to a "Class" number and are based on the number of contaminating particles tolerated in a given volume of air. The lower the Class number, the cleaner the room. Generally, the cleanest of the clean rooms today have a designation of Class 1.
The various semiconductor fabs throughout the world generally follow a fairly standard design in the layout of the clean room facility. A manufacturing area of the fab is designated to have the cleanest environment in order not to contaminate the various devices being fabricated on the semiconductor wafers. This manufacturing area has controlled access in order to reduce the amount of human presence, which can be a significant contributor of contamination in a fab. To maintain this high level of cleanliness, the manufacturing area is designed to have a zone ("room") above the ceiling and a zone below its floor. Essentially, the floor of the lower zone is at ground level and the floor of the manufacturing area is suspended at a predetermined height above the ground level. Thus, the manufacturing area appears to, be a fully contained enclosure suspended above a floor with an open area above the ceiling.
Additionally, equipment and instruments necessary for the distribution of liquids and gases to the various wafer processing equipment are located outside of this contained enclosure and piped in above or below the enclosure to various processing equipment located in the manufacturing area. A containment area, such as a trench, is typically constructed in the lower zone to collect any liquids which may spill from the manufacturing area residing above.
All or portions of the floor and ceiling panels of the clean room have small openings for the passage of forced air. Typically, air is forced downward from the upper zone above the ceiling, through the clean room, out through the suspended floor and into the lower zone. This air is then filtered, recycled back up to the upper area and forced downward again. Because the filtered air in the clean room is replaced a number of times each minute, the manufacturing area is relatively free of contaminating particles. The degree of cleanliness will depend on the amount and type of air filtering used and the controls placed on the generation of contaminants within the manufacturing area.
Due to the ease in which contaminants can be introduced or produced in this clean environment, a number of schemes have been devised to reduce the presence of contaminants in the actual area where wafers or devices are present. Foremost is the restrictions placed on human access. For those entering a clean area, a rigorous entry procedure must be followed, including a dressing procedure in which individuals put on clothing commonly known as "bunny suits." One of the causes of contamination in the clean room is due to the presence of humans in the clean room. Thus, even with the utmost care in the use of the equipment, chemicals etc., humans continually introduce contaminants into the fab.
Other means of controlling the presence of contaminants is by the segregation or isolation of contaminating sources. For example, one technique is to have two or more zones of cleanliness. The wafers are kept in the cleaner (or cleanest) zone, while sources which do not come in contact with the wafer are kept in the less cleaner zones. Thus, maintenance corridors and support areas within a fab are isolated from the wafer processing areas of the clean room.
A more advanced method currently uses a SMIF (Standard Mechanical InterFace) technique to isolate the wafer from the environment. Generally, this technique requires the wafers to be encased in clean enclosures when being stored or transported. Although this technique helps to reduce the amount of contamination, the wafers are always susceptible to contamination since it is difficult to maintain the cleanliness integrity of the enclosures. A state-of-the-art integrated circuit manufacturing facility, generally showing the current design and use of a clean room, is described in U.S. Pat. No. 5,058,491.
Another factor for consideration is from a materials handling viewpoint. Generally, the existing fabs were designed with a central consideration for human access. Equipment location and facility support were then designed in to the fab with constant regard for human access to the equipment. A wafer management system was implemented in order to "move" the wafers through the fab. The process flows of these wafers are generally linear and have resulted in inefficiencies which are characteristic of linear process flow systems.
One solution for overcoming a linear process flow system is to devise a circular system. One such implementation of a circular system is to devise a circular fab. An early design philosophy for a circular fab is described in "Wafers In The Round: A Fresh Approach To Fab Design And Operation", Hinsley, Microcontamination, pp 12-15, June 1986. In this approach the control computer and wafer storage are located at the hub. The various processing equipment are then located adjacent to the spines radiating from the hub.
A more advanced design for a circular fab is described by Professor Tadahiro Ohmi in "Break Through For Scientific Semiconductor Manufacturing In 2001", A Proposal from Tohoku University, Special Issue on 2001 Semiconductor Manufacturing, 1992. Professor Ohmi describes a circular centered single-column fab for production of 220,000 wafers/month in a clean room having an area of 20,000 square meters. This design describes a central transport tower surrounded by process equipment and tools located in the clean area. "Spines" or tunnels are used to transport wafers to the equipment. A piping zone is located below the clean room, while air filters are located above the ceilings. Filtered air flow provides for laminar air flow from the ceiling in order to keep the equipment area ultra clean. The air return under the clean room floor provides for recirculation of the air in the fab.
Although the design is circular and the transport tower is centrally located, the Ohmi design still relies on a number of existing practices which are based on the current premise relating to clean room philosophy. That premise being that the clean room is one large production floor where human access is vital for proper operation and management of the equipment. Thus, in order to isolate the wafer from surrounding contamination, transport tunnels are utilized to transport the wafers from the central tower to the various processing equipment on the clean room floor.
The present invention is a radical departure from the existing practice of designing a fab around human access. The present invention places the paramount importance upon the management of the wafer and provides for a fab which attempts to remove human access from the actual wafer processing areas. Although the present invention relates to the concept of a circular fab, the modularity of the fab, as well as its multi-story structure, provides for a means of maintaining an important control over the manufacturing environment.